The independent variable for the Drops on a Penny lab experiment is the type of solution used for the experiment. The independent variable is the portion of the experiment that is changed between tests.
For the Drops on a Penny lab experiment, testers use an eyedropper to drop single drops of water onto a penny, and they count how many drops can fit on the surface before spilling over. The purpose of the experiment is to test the cohesion and surface tension properties of the penny and solution. Testers can hypothesize as to which solution works best on the surface of the penny and then repeat the experiment with different solutions to view the results.